1. Field of the Invention
The invention relates to the field of electromagnetic interference (EMI) suppression and digital coding techniques used to suppress the interference.
2. Prior Art
Some digital data encoding techniques uses three signal levels to encode two digital states. The bipolar format used in "T1" uses a zero signal level to represent a binary 0 and a positive or negative level to represent a binary 1. The positive and negative levels are alternated to ensure a zero DC level independent of the data which is encoded. The properties of this carrier system as well as others, are discussed in Digital Transmission Systems, by David R. Smith, published by Van Norstrand, Reinhold Company (1985) particularly in Chapter 5 entitled: "Baseband Transmission".
In a high bit rate bipolar system, when certain code sequences are transmitted such as a string of binary ones, electromagnetic interference (EMI) occurs which may exceed set regulatory levels. This is particularly true since EMI is often measured over relatively narrow frequency bands.
One technique to reduce EMI is to scramble the data prior to transmission and descramble it at the receiver. This scrambling can take the form of exclusively oring the data stream with a pseudorandom code sequence. To descramble, the received signals is exclusively ored with the same pseudorandom code sequence used for scrambling. This of course requires synchronization between the two pseudorandom code sequences. This is sometimes accomplished by using a predetermined algorithm which is synchronized at the transmitter and receiver.
As will be seen, the present invention provides EMI suppression particularly for an encoding mechanism employing three distinct signal characteristics, such as three levels, by randomizing the transmitted signal. However, as will be seen this randomizing does not alter the decoded data signal. Consequently, no descrambling is needed.